This invention was made with U.S. Government support under contract number 70NANB8H4022 awarded by NIST. The U.S. Government has certain rights in the invention.
The invention relates to high voltage generators and other dynamoelectric machines having stators with cable windings.
High voltage generators produce electric power at transmission line levels, which are generally between 45 kilovolts (kV) to 750 kV. These generators can connect directly to transmission lines. They avoid the need for step-up transformers that are required for generators that produce only lower voltage levels. High voltage generators use windings formed of round conductive cables instead of the winding bars typically used in lower voltage generators. These round cables are able to support the high voltages and carry the currents produced by high voltage generators.
The round cables used as windings in high voltage generators are insulated with a thick sheath formed of a cross-linked polyethylene or similar insulating material. The sheath protects and insulates the conductor core of the cable. In addition, the cables typically have semiconductor layers on the conductor core and on the outer sheath surface of the cable. These semiconductor layers maintain uniform voltage stresses across the core and sheath surface of the cable winding. The cables, especially their insulation and semiconductor layers, are fragile and should be handled carefully as the cables are inserted into the stator during assembly of the generator.
The stators are large cylinders that encircle the rotor of a generator. The rotor is coaxial to the stator, and the stator has a cylindrical aperture to receive the rotor. The stators have radial slots that extend from the rotor aperture outward into the stator. The cable windings are mounted in the stator slots. The slots extend from one end of the stator to the other and along the length of the rotor. The slots form a circular array around the rotor. The cable windings loop back and forth through the slots to form electrical paths surrounding the rotor. In addition, the stator sections between these slots are typically referred to as the teeth of the stator.
This invention was made with
Each turn of the cable must be separately and mechanically supported in the slot. The cable cannot withstand the compressive forces of a stack of cable loops in a stator slot. If the cable is stacked in a slot without supports (such as the support provided by bicycle chain type slot), the cable at the bottom of the slot would suffer a broken or crushed insulation sheath due to electromagnetic forces in the slot.
An example of a stator slot layout is disclosed in commonly assigned U.S. Pat. No. 3,014,139, (the '139 Patent) entitled "Direct-Cooled Cable Winding For Electromagnetic Device". The bicycle chain slots of the stator shown in the '139 Patent support the cable in the stator, transfer heat generated by electrical current out of the cables, and protect the cables from mechanical stresses that might damage the cable. The slot shape of the stator has the cross-section appearance of the outline of a "bicycle chain" and is designed to support each loop of the cable in the stator slot without excessively compressing the insulation sheath of the cable.
During assembly, the cables used as windings in high voltage generators have traditionally been threaded through the stators by inserting the cable into one end of the stator and drawing the cable through to the other end of the stator. As it is threaded through a stator slot, the cable extends out of an end of the stator, is looped back towards that stator end, is inserted into another stator slot, and drawn through that other slot. The threading of the winding cable back and forth through the stator continues as the cable is inserted into the stator slots. When inserted in the slots, the cable forms the windings for the stator of the generator.
A problem exists in threading cable windings through the stator slots. Conventionally, the cables are inserted at one end of a stator and drawn through a stator slot along the entire axial length of the stator until the cable is pulled through the opposite end of the stator. Threading the cable axially through a stator slot tends to place the cable in excessive tension. The insulation sheath of the cables is fragile, and does not tolerate excessive tension or compression forces. Also, the structural and insulating integrity of the cable surface may be compromised or damaged due to the shearing forces between the cable and stator during cable insertion. The cable must be carefully threaded through the slots of the stator to prevent damage to the insulation sheath. The care in threading the cable through the slot increases the complexity of and time required for the cable threading process. Moreover, the cable is susceptible of being damaged, even when the threading process is carefully conducted.
Another difficulty with the conventional threading technique is that a large room is required to thread the cable back and forth through the stator slots. The cable windings are typically stretched out in a long line as it is being threaded into an end of the stator. Similarly, as the cable is pulled through the stator slot, the end of the cable progressively extends further out along a line from the opposite end of the stator. Accordingly, a large amount of space in a stator assembly room has been needed to thread cable through the slots of a stator.